The trapping and storage of charged particles has been studied for many years. See, for example, L. S. Brown and G. Gabrielse, Rev. Mod. Phys., Vol. 58, pp. 233-311 (1986) and D. H. E. Dubin and T. M. O'Neil, Rev. Mod. Phys., Vol. 71, pp. 87-172 (1999). In addition to basic plasma physics studies, trapped single-component plasmas may have a number of uses including uses in atomic clocks, the tailoring of positron beams for material characterization and the production of low-energy anti-hydrogen. See, for example, J. R. Danielson and C. M. Surko, Phys. Plasmas, Vol. 13, 055706 (2006).
Several traps have been developed for trapping and storing charged particles. For example, Penning traps and, more generally, Malmberg-Penning traps have been developed. A Malmberg-Penning trap is a cylindrically symmetric device that is utilized to confine non-neutral plasmas with both a uniform axial magnetic field that confines the plasma radially and electrostatic fields at the opposed ends of the trap to confine the plasma axially. These traps usually have cylindrical electrodes to generate the electrostatic fields that allow diagnostic access.
In Malmberg-Penning traps, the charged particles within the trap rotate in circles about the magnetic field such that the charged particles are confined radially. The radius r of the circles in which the charged particles rotate is defined as follows:r=(m v)/(e B)where m is the mass of a particle, v is a particle velocity, e is the charge of the particle and B is the value of the magnetic field. The velocity of the particles is generally determined by the temperature of the surroundings since the charged particles moving in the circular rotation exchange heat with the surroundings via radiation. At a temperature of 4 K, for example, the thermal velocity of an electron is about 8 km/s and the radius in a 2 T magnetic field is about 44 nm.
The charged particles may experience a small drag torque while moving in the circular rotation. Although not wishing to be bound by any particular theory, the drag torque may be caused by azimuthal inhomogeneities in the magnetic and electric fields of the trap. The drag torque disadvantageously causes the radius of the circular rotation pattern of a charged particle to increase such that the position of the charged particle migrates towards the walls of the trap. Ultimately, the charged particle may collide with the wall of the trap and the charged particle may escape from the trap. As a result, the lifetime of the charged particles within the trap may be less than desired in some instances. In order to counter the drag torque, some traps apply a rotating electric field that applies torque in the opposite direction to the drag torque. Unfortunately, the application of the rotating electric field adds to both the cost and complexity of the resulting trap.